A massive winter storm that promises to blanket a huge swath of the nation in crazy weather, includes 18 inches of snow or more in Kansas to thunderstorms and flooding along the Gulf Coast.

In all, about 800,000 square miles in 18 states were under some form of watch or warning Wednesday related to a major storm pouring out of California and into the Central Plains and the Midwest, CNN meteorologist Brandon Miller said.

One of the places that could be hit the hardest is Springfield, Missouri. Among the hazards expected there Wednesday: lightning, wind and ice, with a little snow and sleet thrown on top for good measure.

The biggest threat of heavy snow lies in parts of Kansas, Nebraska and Missouri, with the possibility of whiteout conditions in some places, he said.

In Kansas, where the storm had already started, the National Weather Service was predicting as much as 18 inches of snow. The agency said parts of Nebraska could see more than a foot of snow, with drifts making some roads impassable.

Forecasters were predicting as much as two inches of snow in St. Louis, with the precipitation turning into sleet and freezing rain Thursday. The conditions will have "a significant impact on travel," the weather service warned.

North of where the most snow will fall, Chicago could receive what would be its largest single snowfall of the year -- 4 inches, Miller said. The city usually gets up to 4 feet of snow a year. So far this year, only about a foot has fallen, he said.

South of the snow belt, dangerously icy conditions are possible in parts of northern Arkansas and southern Missouri, he said. That includes Springfield, where windy conditions will precede the possibility of up to 3 inches of snow and sleet Thursday, with the possibility of a quarter of an inch or more of ice to boot.

Elsewhere, severe thunderstorms moving in from the Gulf of Mexico are expected to dump from 2 to 6 inches of rain over New Orleans, Louisiana, and Montgomery, Alabama, according to CNN's weather center, before rolling up toward Atlanta.

The torrential rains could lead to significant river flooding near the Gulf, as flood watches are still in effect from last week's heavy rains.

Heavy winds, hail and tornadoes are possible, the National Weather Service says. Downpours are expected to continue into Friday.

Tuesday night, the same storm forced the temporary closure of some stretches of highway northeast of Los Angeles, stranding some motorists until highway crews and police were able to battle through the snow and clear the backlog.

"The snow came in hard and heavy," said Officer Ed Smith of the California Highway Patrol.

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